The invention relates to a tool for producing crown wheels by means of a generating process.
Crown wheels are gear wheels which are used in angle drives with or without intersecting axes Which may or may not form an angle of 90.degree. with each other. In this drive a cylindrical pinion meshes with a crown wheel whose tooth shape is determined by the tooth shape of the cylindrical pinion, the gear ratio and the position of the gear wheels relative to each other.
The angle drive with crown wheel teeth has a number of special advantages over the conical gear drive generally known and used, such as, inter alia, the absence of any need for axial adjustment of the cylindrical pinion, a greater gear ratio being achievable, and a great mesh quotient which is achievable without special provisions.
The lack of an economically feasible, and thus accurate manufacturing method was, however, hitherto one of the greatest disadvantages for a general application of the crown wheel gear in highly loaded constructions. The lack of suitable manufacturing equipment played a major role in this.
The use of hobbing tools, by means of which crown wheels can be produced in a continuous generation process, is in practice a condition for the economic production of crown wheels and for a wider application of crown wheels in highly loaded and/or high-speed constructions.
A hobbing cutter for the production of crown wheels by means of a generating process is known from U.S. Pat. No. 2,304,586 (Miller). This cutter comprises a disc-shaped cutter element with cutting teeth provided on the periphery. The cutting edges of said cutting teeth lie in a surface of rotation produced by the rotation of an imaginary gear wheel of infinitely small thickness about the axis of the cutter element and simultaneously about its own axis, the imaginary gear wheel on one revolution about the axis of the cutter element rotating about its own axis through an angle which is equal to a whole number of times the pitch angle of the imaginary gear wheel, and the surface of the imaginary gear wheel always extending in the radial direction of the cutter element and at right angles to the path described by the imaginary gear wheel.
The cutting edge of the cutting teeth of this known hobbing cutter is always the shape of the outer contour of a cross-section of a segment of a pinion with which a crown wheel being produced is to mesh. The cutting teeth of the known hobbing cutter are provided with clearance faces in the same way as conventional cutting teeth of a cutter. This means that grinding the cutting edges of cutting teeth which have become blunt produces a change in the shape of the cutting edges of the hobbing cutter. The newly ground hobbing cutter generates a crown wheel which can only mesh in the optimum way with a pinion of which the shape of the outer contour of the cross-section is the same as the modified shape of the cutting edges of the hobbing cutter. This is a great disadvantage for exchangeability, and thus for efficient production and a general use of crown wheels.
A similar problem also occurs with hobbing grinding discs for the production of crown wheels which are the same basic shape as the known hobbing cutter. For, dressing such a hobbing grinding disc will cause the shape of the grinding surface to change, and thereby also the shape of a crown wheel ground with such a grinding disc.